In its outlines, the principle of marking is as follows:
A roll consisting of a polyester film is coated on one face with a fine, metallic or otherwise, marking layer, onto which a hot melt adhesive capable of being activated when hot is then deposited. During the marking operation, a gilding stamp whose relief portions represent the image to be marked is heated and pressed against the marking film and the receiving substrate. Only the portions of the film and of the receiving substrate in contact with and pressed by the relief portions of the gilding stamp are heated, and this permits the adhesive which is activated in these regions to adhere and to bond with the marking foil onto the receiving substrate. At each cycle, the transfer of the marking layer, from the film onto the receiving substrate, takes place when the pressure is released and when, on opening, the press moves the film away from the substrate.
When this process first originated, the metal film was made of gold leaf. Nowadays, very frequently, the tape is metalized under vacuum and gold is replaced with colored aluminum. Various pigments or decorative marking substances are sometimes substituted for the metal film. The manufacturers of these marking films offer the users a range of products whose decorative and highly elaborate effects find countless applications.
All sorts of presses with platens or rolls are equipped for performing this hot marking and include devices allowing the tape to move past intermittently, with a forward motion which can be varied as a function of the length of the pattern to be reproduced, and systems for heating the marking stamp and maintaining its temperature.
The pressure required to produce the transfer of the marking layer onto a receiving substrate of the card type is of the order of 250 kg/cm.sup.2.
In many cases in order to improve the decorative effect and the legibility, repousse work on the substrate, in the marked portions, enables them to stand out in relief. The technique which is employed at the present time gives very good results insofar as the quality produced is concerned, but includes a certain number of major defects due to the basic principle of the process and to the means required to implement it.
These defects can be defined as follows:
a transfer machine, press or heated roll, which is very heavy, relatively cumbersome and representing a major investment,
high cost of the materials to be employed, such as gilding stamps or repousse tools, reflected in the cost of manufacture of the printed matter and very difficult to incorporate in its sale price, chiefly in the very frequent cases of print runs involving few copies,
lack of flexibility in use and excessively long processing time, restricting its development potential, and
the need to employ extremely highly qualified operators, since the latter are often confronted with a set of difficulties to be overcome in order to produce a high-quality marking.
Overall, these difficulties can be listed as follows:
relative uneveness of thickness in different regions of one and the same substrate,
surface quality which varies between one substrate to be marked and another,
pressure phenomena which are very frequently difficult to control fully and chiefly in platen presses, where it is very tricky to obtain a perfect pressure distribution and chiefly when the problem is to treat simultaneously patterns comprising large impression surfaces close to fine lines,
generally mediocre temperature distribution in the heated platens to which the marking blocks are fastened, resulting in defects in the transfer of the marking foil, and
relatively very low hourly production.
Furthermore, and in order to appreciate properly the care required to produce satisfactory marking, what must be known is that the marking film and the very thin layer of hot-melt adhesive with which it is coated both have a thickness of the order of one micron and, in most cases, tolerate only very narrow temperature and pressure deviations. Moreover, the repousse work for producing relief is also a matter for an expert, because this extremely tricky operation calls for reliable experience, chiefly when using marking stamps which enable gilding and repousse work to be performed simultaneously.